1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus that forms an image by employing an inkjet printhead mounted on a carriage, which incorporates a positioning mechanism for positioning the printhead carriage to adjust a gap defined between the printhead and a plane supporting a recording medium during printing.
2. Discussion of the Background
Inkjet printing technologies are employed in various image forming apparatuses, such as printers, facsimiles, photocopiers, plotters, and multifunctional machines incorporating several of those imaging capabilities.
An inkjet printer forms an image by employing a fluid-ejecting device called a printhead having one or more nozzles provided in a nozzle face to eject ink in droplets onto a recording medium such as a sheet of paper. The printhead is typically mounted on a carriage supported on one or more guide members extending along a scanning axis, with the nozzle face directed toward a generally planar, horizontal plane on which a recording medium is placed during printing.
To date, mainly two types of inkjet printers are available, depending on the configuration of printhead employed: a serial inkjet printer with a relatively narrow movable printhead, and a line inkjet printer with a relatively wide stationary printhead. The former prints an image by reciprocally moving the printhead back and forth along the scanning axis to traverse the width of a recording sheet, whereas the latter operates by advancing the recording sheet past the printhead without reciprocating the printhead, which spans the entire width of the recording sheet.
What is important for good performance of an inkjet printer is to establish an adequate distance or “print gap” between the printhead and the plane supporting a recording sheet during printing, or more precisely, to maintain a constant distance between the printhead nozzle face and the surface of a recording sheet placed on the sheet supporting plane. In general, too narrow a print gap causes the nozzle face to interfere with the recording sheet, resulting in ink smearing or blotting the recording sheet and/or causing frequent sheet jams, and too large a print gap reduces the accuracy with which the printhead deposits ink at desired locations on the recording sheet, leading to concomitant image defects, such as misregistration and color inconsistencies, appearing on a resulting image.
To obtain a desired print gap, a printhead assembly is typically provided with a positioning mechanism that enables positioning of the printhead carriage by moving it relative to the sheet supporting plane. For example, one such mechanism uses a pair of eccentric bearings mounted to opposite ends of an elongated guide member supporting a printhead carriage over a sheet supporting plane, which are rotatable around a rotational axis offset from a longitudinal axis of the guide member. When rotated, these eccentric bearings cause the guide member to move perpendicular to its longitudinal axis together with the carriage supported thereon. The positioning mechanism is provided with an operating lever that enables an operator to adjust the print gap by raising or lowering the carriage relative to the sheet supporting plane.
Such print gap adjustment is particularly important where an inkjet printer handles various types of recording sheets from one operation to another. This is because an appropriate print gap depends on the type of recording sheet in use, which has a specific thickness to define a spacing between the nozzle face and the sheet surface during printing. In other words, to maintain a constant space between the nozzle face and the sheet surface, wider print gaps are needed when thicker recording sheets are used, and narrower print gaps are needed when thinner recording sheets are used. Thus, a positioning mechanism is required to accommodate variations in the print gap depending on the thickness of recording sheet.
A straightforward approach to meeting this requirement is to vary the carriage position and the print gap steplessly depending on the recording sheet thickness. Although desirable for accurate spacing between the nozzle face and the sheet surface, such a configuration would complicate the positioning mechanism, making it expensive to manufacture and difficult to handle.
A more practical approach is to provide only two levels of print gaps or operational positions to which the positioning mechanism can position a printhead carriage, one for relatively thick sheets and the other for relatively thin sheets. This approach is simple and ready to implement compared to steplessly varying the print gap, and works well where an inkjet printer normally handles recording sheets of only two standard thicknesses. For such reasons, the two-level positioning arrangement is employed in several printhead positioning mechanisms.
However, this conventional method has certain drawbacks. One drawback is that the two-level positioning, by its nature, cannot provide an appropriate print gap for any specific recording sheet that has a thickness other than those of standard thick and thin recording sheets. Moreover, the simple two-level positioning fails to accommodate variations in the print gap caused by environmental factors other than sheet thickness, for example, deformation or displacement of a transport belt defining a surface on which a recording sheet is placed during printing.
Another drawback of the conventional method is the difficulty in initially setting up two levels of print gap with both precise parallelism and adequate spacing between the nozzle face and the sheet supporting plane. This difficulty is a problem not just with the two-level positioning but with all the positioning mechanisms that need initial setup of the print gap. Typically, initial adjustment for a print gap is a complicated process requiring some special tools to complete, resulting in considerable time required during assembly of a printhead carriage.
Hence, what is needed is a simple mechanism for positioning a printhead carriage in an image forming apparatus that provides ready adjustment of a print gap to accommodate both various types of recording sheets and environmental factors causing variations in the print gap, while requiring no complicated process to establish an appropriate print gap during assembly of the printhead carriage.